The standard “DIN 53519-1 Testing of elastomers - Determination of indentation hardness of soft rubber (IRHD) - Hardness test on standard specimens”, developed by the German Institute for Standardization (DIN), specifies materials and their hardness and provides information on the application of O-rings covered in the standard “ISO 3601-1 O-rings for use in fluid systems - Dimensions”.
The Shore A scale is most commonly used to measure the hardness of rubbers and can be determined using a durometer gauge. The durometer consists of a metal truncated cone indenter attached to a small spring. The indenter extends 2,5 mm into the material. The material resists the indentation by deflecting the spring. Every 0,0254 mm that the spring is deflected is related to 1 degree of Shore A. Harder materials resist indentation, causing the spring to deflection to be greater, resulting in a higher reading. Shore 70 A is the standard hardness for rubber.
The great thing about the Shore hardness scale is that it can measure any material. This scale is tailored to meet the needs or requirements for any material measurement. The Shore A scale is generally used to measure softer rubbers and the Shore D scale is used for harder ones. The Shore D scale was developed to measure the hardness of harder rubbers that have a value above 90 degrees on the Shore A scale.
Similarly, Shore 0 was introduced to measure the hardness of soft rubbers that register less than 10 degrees on the Shore A scale. Shore 00 was introduced for very soft rubbers or gels that register less than 0 degrees on the Shore 10 scale. Shore B and Shore C are intermediate scales designed for use on medium hardness rubbers.
Shore hardness testing usually leaves a permanent indentation in the test specimen. Another consideration is that the material must be at least 6 mm thick and placed on a hard surface to obtain an accurate reading. If the material is very thin, the durometer measures the hardness of the surface on which the material is placed. Due to the mechanical limitations of testing instruments, hardness measurements on elastomers are normally expressed in 5 degree increments.
Like Shore hardness, the International Rubber Hardness Rating (IRHD) test results in measurements between 0 and 100. The difference is that the value on the IRHD scale is not linear and is related to the material's Young's Modulus (modulus of elasticity in tension or compression). The IRHD scale is chosen so that a material with a Young's Modulus of zero is represented by an IHRD hardness of 0, and a material with an infinite Young's modulus corresponds to a hardness of 100.
There are four methods of performing the test, depending on the hardness range of the rubber:
International Rubber Hardness (IRHD) is measured by plunging a spherical ball into a sample of material under two different loads. The difference in indentation depth under each load is measured.
IRHD is a non-destructive method of testing hardness and does not leave an indentation in the rubber. IRHD micro testing can be performed on materials from 1 to 5 mm thick. Micro testing is an ideal method for testing thin products such as O-rings. This is beneficial as it will give a hardness value for the specific product, not just the material used. It has been found that the shape of the material does not greatly affect the overall hardness of the product.
Both the IRHD and Shore scales give hardness measurements on a scale of 0 to 100 degrees. The difference between these two methods is that the Shore scales are linear based on the deflection of the rubber, while the IRHD is a non-linear scale related to the elastic modulus of the material. For highly elastic rubbers, Shore A and IRHD give comparable values.
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